Patent Literature 1 describes that when a liquid refrigerant amount accumulated in a compressor becomes a predetermined amount or more, a weak high-frequency open-phase current is caused to flow to a motor winding to warm the motor winding. Accordingly, liquid compression due to startup in a state with the liquid refrigerant being accumulated in the compressor is prevented to prevent damage of the compressor.
Patent Literature 2 describes that a direction of a current flowing to a stator coil of a motor is cyclically reversed by controlling an on/off cycle of a switching element. Accordingly, not only heat generation by an ohmic loss but also heat generation by an hysteresis loss is performed so that sufficient preheating can be performed with a less consumption current, thereby improving power efficiency.